Its more the update circuitry that's responsible - it row-scans like an analog TV. Combined with the relatively slow eInk update rate you get the very-large product of a second per frame or so. There is nothing inherent in eInk that demands row-scanning; its just the choice made by the first commercially-available eInk screen.

Patents exist for better eInk displays: http://www.faqs.org/patents/app/20080238894 I look forward to a fast-updating book reader. One day I may even be able to flip through an eBook! Until then they have a long way to go, to reproduce the book-in-hand feel.